Many men who have sex with men (MSM) choose their sex partners, selectively use condoms or alter their sexual repertoire based on a partner's or potential partner's HIV status. How these seroadaptive behaviors affect a man's risk for acquiring HIV or other sexually transmitted diseases (STD) is unknown. Some data suggest that selectively engaging in unprotected anal intercourse (UAI) based on partners concordant HIV status (selective UAI) offers men partial protection from HIV acquisition, but these data are based on subjects' reports of their sexual behavior. However, decisions to seroadapt are only one factor affecting sexual behavior, and knowing if seroadaptation is protective requires actually measuring its impact. We will conduct cognitive interviews to refine a data collection instrument on seroadaption, and then integrate that instrument into a computer assisted self-interview (CASI) currently used by all STD clinic patients in Seattle, WA. The CASI will collect data on seroadaptive behaviors from consenting MSM. We will measure the association of respondents' seroadaptive decisions with their sexual behavior, and hypothesize that these outcomes will be highly associated, but that agreement between them will only be moderate (kappa=0.41-.6), confirming the need to measure seroadaptation when studying MSM's behavioral prevention strategies. Analyzes will use generalized estimating equations to evaluate the association of seroadaptive behaviors with HIV and STI outcomes. We hypothesize that: 1) selective UAI will be associated with a lower risk of HIV among MSM overall, but an elevated risk of HIV compared to consistent condom use or not having anal sex; 2) selective UAI with primary partners will be associated with a lower risk of HIV than selective UAI with casual or nonprimary partners; 3) selective condom use for receptive UAI will be associated with a higher risk of HIV than consistent condoms use, but a decreased HIV risk after controlling for selective partnership formation and selective engagement in anal (vs. oral or manual) sex; 4) the majority of MSM with newly diagnosed HIV will report selective UAI, and 5) selective UAI will be associated with an elevated risk of syphilis or urethral/rectal gonorrhea or chlamydia among HIV infected MSM. PUBLIC HEALTH RELEVANCE: Men who have sex with men (MSM) commonly choose sex partners, use condoms or otherwise alter their sexual behavior based on their sex partners' percieved HIV status. The proposed study will define how much these alterations in behaviors protect MSM from HIV. This information will allow clinicians and prevention workers to more accurately counsel MSM about how to reduce their risk, and will help researchers identify which changes in behavior should be prioritized in behavioral interventions designed to decrease HIV risk.